An apparatus has been built for the servo-control of workingcontractions of isolated frog skeletal muscle within an NMRspectrometer. 31P and 1H NMR spectra are obtained of repetitivelycontracting muscle. Recent effort has focused on identifying thetwo peaks which appear in the lactate methyl region of fatiguingmuscles. A variety of biochemical and NMR evidence has lead to theconclusion that both peaks are lactate, and that the lactate fromoxidative fibers is shifted by a susceptibility effect